drm/i915: Move some hdmi enable function name to vlv specific.

There is no functional change on this patch.  Only rename several
hdmi encoder function name which suppose to use only by valleyview from
intel_hdmi_pre_pll_enable to vlv_hdmi_pre_pll_enable, and etc.

Signed-off-by: Chon Ming Lee <chon.ming.lee@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>